krahets/hello-algo · error

キューが空です

Error message

キューが空です

What it means

This panic is raised by peek() in a singly-linked-list queue (Japanese localization). It fires when reading the front element while que_size == 0. The guard prevents a null front dereference. @panic aborts and is uncatchable.

Source

Thrown at ja/codes/zig/chapter_stack_and_queue/linkedlist_queue.zig:48

        // デストラクタ（メモリを解放する）
        pub fn deinit(self: *Self) void {
            if (self.mem_arena == null) return;
            self.mem_arena.?.deinit();
        }

        // キューの長さを取得
        pub fn size(self: *Self) usize {
            return self.que_size;
        }

        // キューが空かどうかを判定
        pub fn isEmpty(self: *Self) bool {
            return self.size() == 0;
        }

        // キュー先頭の要素にアクセス
        pub fn peek(self: *Self) T {
            if (self.size() == 0) @panic("キューが空です");
            return self.front.?.val;
        }  

        // エンキュー
        pub fn push(self: *Self, num: T) !void {
            // 末尾ノードの後ろに num を追加
            var node = try self.mem_allocator.create(inc.ListNode(T));
            node.init(num);
            // キューが空なら、先頭・末尾ノードをともにそのノードに設定
            if (self.front == null) {
                self.front = node;
                self.rear = node;
            // キューが空でなければ、そのノードを末尾ノードの後ろに追加
            } else {
                self.rear.?.next = node;
                self.rear = node;
            }
            self.que_size += 1;

View on GitHub (pinned to 69932aed18)

Solutions

1. Check queue.isEmpty() (or size()==0) before peek().
2. Drive the consumer with while (!queue.isEmpty()).
3. Cache size locally and branch before reading front.
4. Wrap peek in a helper returning ?T for recoverable use.

Example fix

// before
var head = queue.peek(); // @panics when empty

// after
while (!queue.isEmpty()) {
    var head = queue.peek();
}

// Validate non-empty before reading the front
if (!queue.isEmpty()) {
    var head = queue.peek();
}

When it happens

Trigger: Calling queue.peek() on a new queue, or after the last element has been dequeued.

Common situations: BFS frontier peek after draining; producer/consumer where consumer reads ahead; porting catchable-exception queue code from another language.

Related errors

• キューが空です
• 队列为空
• スタックが空です
• 両端キューが空です
• スタックが空です

AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13). Data as JSON: /api/errors/b33f461c2dcde8a8. Report an issue: GitHub.